There has been considerable recent interest in the subject of elevated temperature molten liquid metal batteries for stationary electrical storage. This low cost energy storage technology holds promise to safely store highly intermittent renewable energy sources such as solar and wind energy. This will help meet increasing global electricity demand while simultaneously reducing CO2 greenhouse emissions. These rechargeable (secondary) batteries consist of an anode, a cathode, and a suitable electrolyte salt that provides ionic conduction between them.
In molten metal rechargeable electrochemical cells, the cathode, the anode, and the electrolyte salt are stacked in order of decreasing liquid densities and heated to a molten state. When each of these three components is molten, their differing densities cause them to naturally settle into three, separate, horizontal, self-segregating (self-assembling), non-miscible molten levels. In this density stratification arrangement, the lower density molten metal electrode floats on top; the higher density molten metal electrode sinks to the bottom, and the electrolyte salt, having an intermediate density, floats between the two molten metal electrodes. Since all active components are molten, electrochemical diffusion and ion transport reactions are enhanced. And, molten metal electrodes are not susceptible to mechanical failure that often accompanies solid electrodes in electrochemical cells. This generally leads to an extended cell cycle life. The fact that these electrodes are in their molten states also precludes the formation of dendrites.
Traditional molten metal rechargeable electrochemical cells are described with alkaline or alkaline earth metal (e.g. magnesium) used as the less dense anode metal and antimony, Sb, used as the more dense cathode metal. These elevated temperature Mg—Sb cells use a suitable, ionically conducting electrolyte salt. When these components are heated to a temperature of greater than or equal to their respective melting temperatures, stratification of each of these species occurs. The denser molten Sb cathode settles to the bottom, the molten salt electrolyte (MgCl2—KCl—NaCl) rests in the middle, and the less dense molten Mg anode floats on top. Adjacent molten layers are immiscible in each other. During a cell discharge reaction, at the anode, Mg metal is oxidized to form an Mg2+ cation that migrates through the molten salt electrolyte to the Sb cathode where it is reduced to form neutral Mg that alloys with the molten Sb. As Mg is incorporated into the bulk of the Sb cathode structure, it forms an Mg—Sb alloy and the value of the subscript x in the formed metal alloy Mg—Sb gradually increases. During electrochemical recharge, when electrical current flows into the cell, Mg is driven out of the Mg—Sb alloy, across the electrolyte, where it then forms the original Mg anode.
The overall cell voltage during discharge is due to the differences in chemical potential between the metal anode when it is in its pure metal state and when this anode is alloyed with the cathode metal.
These traditional molten electrode electrochemical cells suffer from a number of substantial technical drawbacks. For instance, the anode material, the cathode material, or both comprise materials, such as magnesium, that react violently with oxygen or air at elevated temperatures. Therefore, these cells require electrical heating elements and hermetically sealed housings that restrict air or oxygen from entering into the housing of the cell and reacting with the molten metals therein. In turn, this necessitates complicated and inefficient cell configurations that are required to form useful batteries. Moreover, technical problems (e.g., effective sealing and anti-corrosion measures) arise from the manufacture of cells that are airtight at elevated temperatures. Solutions to these technical problems add to system design complexity and increased cell construction costs. Traditional molten electrode cells are also limited by reduced performance characteristics. For example, many traditional molten cells generate undesirably low voltages (e.g., less than 1 V). And, in other traditional cells, the anode material, the cathode material, or both comprise environmentally harmful, toxic, or otherwise expensive metals.